Low activation threshold as a mechanism for ligand-independent signaling in pre-T cells.

Pre-TCR complexes are thought to signal in a ligand-independent manner because they are constitutively targeted to lipid rafts. We report that ligand-independent signaling is not a unique capability of the pre-TCR complex. Indeed, the TCRalpha subunit restores development of pTalpha-deficient thymocytes to the CD4(+)CD8(+) stage even in the absence of conventional MHC class I and class II ligands. Moreover, we found that pre-TCR and alpha,betaTCR complexes exhibit no appreciable difference in their association with lipid rafts, suggesting that ligand-independence is a function of the CD4(-)CD8(-)(DN) thymocytes in which pre-TCR signaling occurs. In agreement, we found that only CD44(-)CD25(+) DN thymocytes (DN3) enabled activation of extracellular signal-regulated kinases by the pre-TCR complex. DN thymocytes also exhibited a lower signaling threshold relative to CD4(+)CD8(+) thymocytes, which was associated with both the markedly elevated lipid raft content of their plasma membranes and more robust capacitative Ca2+ entry. Taken together these data suggest that cell-autonomous, ligand-independent signaling is primarily a property of the thymocytes in which pre-TCR signaling occurs.